Conventional small-size unmanned aerial vehicles represented by industrial unmanned helicopters have had airframes too expensive to be affordable. Also, these vehicles used to require skillful pilotage for stable flight. In recent years, however, there have been improvements and cost reductions in sensors and software used to control posture of unmanned aerial vehicles and to implement autonomous flight of unmanned aerial vehicles. This has led to considerable improvement in manipulability of unmanned aerial vehicles. In particular, small size multi-copters are simpler in rotor structure and easier in design and maintenance than helicopters. Under the circumstances, small size multi-copters are not only used for hobbyist purposes but also applied to various missions in a wide range of fields.